1. Field of the Invention
The present invention generally relates to charged particle beam exposure methods and apparatuses, and more particularly to a charged particle beam exposure method which carries out the exposure with respect to a substrate placed on a stage while continuously moving the stage, and to a charged particle beam exposure apparatus which employs such a charged particle beam exposure method.
2. Description of the Related Art
Recently, the integration density of integrated circuits (ICs) has further improved, and the integration density of ICs have improved to approximately four times over the past several years. For example, in dynamic random access memories (DRAMs), the storage capacity is being improved from 1M to 4M, 16M, 64M, 256M and even 1 G by the improvement in the integration density. The improvement in the integration density was realized mainly by the progress made in the technique used for forming fine patterns. In other words, due to the progress made in the photolithography technique, it has become possible to expose fine patterns on the order of 0.5 .mu.m.
However, when the photolithography technique is used, the limit of the fine pattern that can be exposed is approximately 0.4 .mu.m. In addition, when forming windows for contact holes and making an alignment with respect to an underlying layer, it is extremely difficult to guarantee an accuracy of 0.15 .mu.m or less using the photolithography technique.
On the other hand, much attention is drawn to exposure techniques using a charged particle beam typified by an electron beam. It is expected that the charged particle beam exposure technique can realize the exposure of finer patterns and the highly accurate alignment at a high speed with a high reliability when compared to the photolithography technique.
In the case of the electron beam exposure, when the exposing patterns include dense and coarse portions, the exposure time becomes long at the dense portion and short at the coarse portion. In other words, the exposure speed becomes high or low depending on whether the exposing pattern is coarse or dense. For this reason, when a substrate to be exposed is placed on a stage and the stage is continuously moved, it is necessary to set the moving speed of the stage depending on the exposing patterns. That is, the stage moving speed is set low for the dense portion of the exposing pattern, and high for the coarse portion of the exposing pattern. As a result, the exposure time cannot be reduced sufficiently in the conventional charged particle beam exposure method which moves the stage continuously.
In order to solve the above described problem, an improved charged particle beam exposure method was previously proposed in a Japanese Laid-Open Patent Application No. 5-90142.
According to this previously proposed method, a large deflection range of an exposure region is divided into a plurality of band-shaped regions each extending in a direction perpendicular to the moving direction of the stage, and the exposure time is calculated for each band-shaped region. The large deflection range refers to a range in which the charged particle beam can be deflected within the large deflection range by a large deflector. In addition, a plurality of band-shaped regions arranged along the stage moving direction is defined as a cell region, and the moving speed of the stage that enables the exposure in the cell region is calculated. Furthermore, in order to enable movement of the stage in the cell region at the stage moving speed which makes it possible to carry out the exposure within the exposure time, a frame region is defined as being made up of a plurality of cell regions arranged along the stage moving direction, and 16 speed changing points and speeds are set for this frame region. The exposure is carried out while changing the stage moving speed depending on the set speed changing points and speeds.
Therefore, when actually carrying out the exposure, the exposure time is calculated for each band-shaped region, and the exposure speed is determined based on the calculated exposure time. For this reason, the exposure time, that is, the stage moving speed, is variably set depending on the coarse and dense portions of the exposing patterns, thereby reducing unnecessary movements of the stage at the low speed.
A description related to the actual operations that are carried out when determining the stage moving speed in the previously proposed method will be omitted in this specification.
When the stage starts to move at the stage moving speed which is determined in the above described manner, a stage controller samples the amount of deflection of the large deflector (hereinafter simply referred to as a large deflection quantity) for every 1 ms, so as to continue monitoring the drawing state, that is, whether the stage moving speed is too fast, too slow or appropriate with respect to the specified drawing range. If the large deflection quantity exceeds the drawable region, the stage controller recognizes the stage moving direction, and judges whether the stage movement should be accelerated or decelerated. Hence, the patterns are drawn while variably controlling the stage moving speed.
However, when drawing within the frame region, not only the drawable state but three other states may occur. The three other states will hereinafter referred to as a beam OFF state, a non-reached state, and a passed state.
In the beam OFF state, the exposure beam is OFF. In this beam OFF state, the large deflection quantity at the time when the beam is OFF is held when making the stage control. For this reason, it is possible to control the stage to accelerate, decelerate or move at a constant speed depending on the setting of the value of the large deflection quantity that is held. Normally, when carrying out the exposure by continuously moving the stage, the exposure beam is turned OFF only at the start and end positions of the frame region. Accordingly, in the beam OFF state, the large deflection quantity is set within the drawable range so that the stage moves at the constant speed.
In the non-reached state, the exposure beam has not reached the drawable range. Unless in the drawable state, the large deflection quantity in the drawable state where the exposure beam is OFF at the previous stage remains held in the non-reached state. For this reason, the stage control in the non-reached state is carried out similarly as in the beam OFF state.
In the passed state, the exposure beam has passed the drawable range. In this passed state, a target value for drawing the next cell region is updated, and the drawing is started again. The large deflection quantity in the passed state corresponds to the drawable range in the stage moving direction, and the stage is decelerated by the stage control.
Therefore, in the previously proposed method, a host computer conjectures in advance the large deflection quantity while monitoring the drawing state, and variably controls the stage moving speed based on this conjecture.
But according to the previously proposed method, the number of changing points where the stage moving speed can be changed within the frame region is limited to a constant number regardless of the number of cell regions. More particularly, the number of changing points is only 16. For this reason, when the number of chips within the frame region increases, there was a problem in that the previously proposed method cannot cope with fine changes in the stage moving speed. In other words, after the stage moving speed is changed by a fixed number of times (16 times) within the frame region, the stage moving speed for the remaining parts of the frame region had to be set to the fixed slow speed that conforms to the most dense pattern.
In addition, when controlling the stage to accelerate or decelerate in the drawing state, the host computer had to control the large deflection quantity for each of the states which may occur during the drawing of the patterns and are other than the drawable state, that is, for each of the beam OFF state, the non-reached state and the passed state. As a result, there was also a problem in that the processing time associated with the control of the stage moving speed increases.
Furthermore, it is necessary to judge the drawing state in the stage controller based on the large deflection quantity. For this reason, there was a problem in that the processing time associated with the control of the stage moving speed also increases due to this judging process.
Because of the increased processing time, the load on the host computer and the stage controller is large when the previously proposed method is employed. Therefore, it was impossible to finely and accurately control the stage movement.